The development and maintenance of neural circuits is essential for the formation and function of the nervous system. Circuit development entails axon guidance, synaptic target selection, synapse formation, and synaptic growth and remodeling in response to developmental and environmental inputs. Disrupting these processes in childhood can lead to neurodevelopmental disorders such as mental retardation and autism. In the adult, plasticity mechanisms inducing circuit remodeling likely allow for the encoding of memories, but also contribute to the development of chronic pain and drug addiction. While circuit plasticity is important, it is also essential that circuits be maintained. Disease, trauma,and neurotoxins, including medicines such as chemotherapeutics as well as drugs of abuse such as alcohol and ecstasy, can all trigger an ill-defined axonal degeneration cascade that leads to axonal loss with profound consequences for neurological function. Inhibiting this degeneration program has tremendous potential for the treatment of a host of devastating neurological disorders including Parkinson's disease, Multiple Sclerosis, and neuropathies, but the critical barrier in the field is the lack of knowledge of the key components of the degeneration pathway. Remarkably, a single molecule, the ubiquitin ligase Phr1/Highwire, is a central regulator of both these developmental and degenerative axonal responses. This application explores the function of the Phr1/Highwire ligase, focusing on the recent finding that it is required for normal axonal degeneration in the mouse. The therapeutic potential of inhibiting Phr1 is tested in vivo, while in vitro neuronal culture experiments explore the mechanism of the Phr1-dependent degeneration program. In addition, new Drosophila mutants have been identified that regulate synaptic development and that interact with highwire and its target the MAPKKK Wallenda/DLK. Powerful genetic techniques will be used to analyze these mutants to define molecular pathways controlled by this ligase, allowing for integration of findings in fly and mouse. This innovative, multi-system approach will generate insights into the mechanisms by which neuronal circuits develop and degenerate, and test the therapeutic potential of targeting this essential, evolutionarily conserved ligase and its interaction partners for the treatment of a host of neurological disorders characterized by axon loss.